Microdots are text micronized into, usually, a round dot, typically the size of a period or the dot at the top of say an i or j.
It was in Paris, 1870, France was under siege, when photographer René Dagron used a photographic technique to make important documents small enough for the pigeons to carry. It was 1925 when Emanuel Goldberg came up with a two step process to make the dots smaller, down to 0.1 square mm. This density is comparable to the entire text of the Bible fifty times over in one square inch. Microdots have been used for wartime purposes as well as espionage.